The invention relates generally to meters having indicating pointers, and more particularly, to a system for detecting the location of the pointer.
There has been devised many types of meters having means to detect the position or movement of the pointer, however, most suffer from one or more detriments as a result of the detecting means. Basically they can be broken down into broad categories as contacting and non-contacting type. In the contacting type of detecting means, the needle makes physical contact with the detecting means and such contact may make or break an electrical switch so that the needle contact is sensed.
The contact type of detecting systems generally have the inherent problem that the physical contact in some way affects the otherwise free movement of the pointer. In most sensitive meters, a great deal of stress is put on the lack of friction in the movement to minimize any forces which could impede free movement of the pointer to insure that, to the extent possible, the pointer movement precisely responds to the particular parameter that the pointer is sensing.
In the non-contact type of detecting system, some means is employed such that there is no physical contact between that means and the pointer. Most popular of the non-contacting type detecting systems include magnetic pick-ups and photoelectric devices. It is the latter method to which the present invention pertains.
Prior art detecting systems of the photoelectric type have included various means. For example, one such system includes an individual light source and individual light detector. The light source is aimed at a particular position and, as the pointer passes that position, the light is reflected off the pointer to the light detector. A difficulty, of course, of such detecting system is the lack of easy adjustment in the event some other pointer position is desired. Both the light source and light detector must then be carefully adjusted such that the new position be sensed. Alternatively, for different positions, one could utilize a plurality of such individual light sources, accurately directed toward a plurality of light sensors.
Obviously, the difficulties of plurality of sensors and detectors increases as does the number of such sensor and detectors as each must be accurately set to the desired pointer location and one still must sacrifice the possibility of sensing a pointer position intermediate an individual light source and light detector. Since the number of such sources, etc. must be finite, the number of detectable positions also is finite.
As a further example of the prior art, other arrangements have placed the light source on the end of the pointer itself such that the light source moved with the pointer and thus could be detected in various positions of the pointer, with the light detector being adjustable to such various positions. A difficulty in such arrangement involves the difficult task of replacing such bulb and in the cost of manufacturing and installing a pointer with a light source at its end. Also, since many gauges having pointers are extremely sensitive, it is disadvantageous to add any weight to the pointer in that it thus adds additional drag to the pointer bearings and can slow the needle movement in responding to a change in the sensed parameter.
Other difficulties inherent in prior art methods when light sources and/or light detectors are used outside the face of the gauge include a lessening of the visibility of the gauge pointer. In instances where the gauge is monitored by a person such that undistorted visibility is a must, the face must be as uncluttered as possible by any interfering devices.